1. Field of the Invention
This invention relates to a hybrid integrated circuit in which a copper plate is formed directly on the main surface of a ceramic substrate, and a method for manufacturing the same.
2. Description of the Related Art
As electronic devices are miniaturized and lightened, they have been provided with a hybrid integrated circuit having a ceramic substrate made of, for example, alumina and a thick wiring layer formed on a ceramic substrate, by means of screen printing and burning. Also, with the increase of the packaging density of the circuit, hybrid integrated circuits having an insulating layer on a ceramic substrate have been developed.
FIG. 2 shows a conventional hybrid integrated circuit in which a thick copper layer is formed on a ceramic substrate. Ceramic substrate 1 is made of, for example, alumina and thick copper layer 2 is formed thereon. Insulating layer 3 is formed on thick copper layer 2, and wiring layer 4 is formed on insulating layer 3. This hybrid integrated circuit is manufactured as follows. First, conductive copper paste is printed on ceramic substrate 1 by screen printing, and burned in nitrogen atmosphere, resulting in thick copper layer 2. Next, insulating paste is printed on said copper layer 2, and burned in a nitrogen atmosphere, forming insulating layer 3, then conductive copper paste is printed on said insulating layer 3 and burned in nitrogen atmosphere, forming wiring layer 4.
The paste used for forming thick copper layer 2 generally contains copper powder, an organic vehicle, a solvent, etc., all of which are mixed together. When this paste is printed on ceramic substrate 1 and burned, cracked/reaction gas 5 is generated inside thick copper layer 2 and, as a result, a void is formed on the junction between copper layer 2 and insulating layer 3, and/or in insulating layer 3. Hence, the withstanding voltage of layer 3 is low, the adhesion strength is small, and a short circuit is produced. Thus, insulating layer 3 does not function sufficiently.
"Hybrid Technology of New Generations", Electronic Material, (May 1984), pp. 51-60, published by Kogyo Chosa Kai Publishing Co., Ltd., discloses a method for directly adhering a copper circuit plate to a ceramic substrate by means of the DBC (Direct Bond Copper) technique, as well as a transistor module formed by that method.
Published Unexamined Japanese Patent Application (PUJPA) No. 52-37914 discloses a method for directly adhering a metal to ceramics or another metal.